We propose to continue our investigations of the electronic and geometric properties of the likely reactants, intermediates, and products of biotinmediated carboxylations, decarboxylations, and transcarboxylations, and at the same time inaugurate our studies of model participants in the biotinavidin interaction. Biotin is a vitamin responsible for the uptake and transfer of carbon dioxide; the enzymes it serves are involved in key metabolic processes such as gluconeogenesis, fatty acid synthesis and propionate elaboration. The biotin enzymes share a number of architectural and mechanistic characeristis; our results will be germane in the understanding of them all. Avidin is a protein found in the whites of most poultry eggs; it binds biotin with a tenacity unparalleled in non-covalent interactions. The biotin-avidin interaction is becoming an extremely important tool in both separation and purification science and in cytochemical and immunological localization procedures. From a mechanistic point of view the biotin-avidin interaction is only poorly understood. In our studies of biotin as coezyme and as a biochemical tool we will investigate, by the techniques of experimental high resolution x-ray diffraction and theoretical ab initio quantum mechanics, the properties of model systems germane to these functions. Both techniques yield detailed information about the geometric and electronic properties of molecules. by looking at individual participants in biotin-mediated processes we can surmise the states of key intermediates prior to interaction. By looking at pairs of participants, either by cocrystallization, by modeling electrostatic interaction from the experimentally determined properties of the individuals involved, or by computation of ensemble properties theoretically, we can surmise the dynamic processes involved in these functions of biotin. At the heart of these studies is a clearer understanding of the mechanisms, and in particular the electronic and geometric details, by which biotin acts.